This invention relates to a head gap adjustment device for automatically adjusting the head gap of a printer.
A printer comprises a platen that supports a print medium, such as a sheet of paper, multi-ply forms or a bankbook, and a print head, disposed facing the platen, for printing characters and symbols on the print medium. The head gap refers to the gap between the print head and the print medium. For best printing, the head gap must be adjusted to a certain optimal size.
The print head of a dot matrix impact printer, for example, has a set of dot wires which are driven forward to press a ribbon against the print medium, thereby printing patterns of dots. For maximum printing speed the gap must be as small as possible to let the dot wires impact the ribbon and print medium as quickly as possible.
To obtain the optimum head gap the position of the print head must be adjusted according to the thickness of the print medium, which may range from about 0.05 mm for a single sheet of paper, to as much as 1.5 mm for multi-ply forms or a bankbook. The adjustment should preferably be automatic, since manual adjustment is inconvenient and difficult to perform accurately.
A prior-art device for automatic head gap adjustment comprises a pulse-driven stepping motor capable of moving the print head forward or backward in relation to the platen. Starting from a fixed position, the print head is driven forward for a fixed number of pulses, then backward for a smaller fixed number of pulses. The forward pulse sequence drives the print head into contact with the print medium, at which point forward motion stops. The forward pulse sequence continues thereafter until the fixed number of pulses is reached, the print head and stepping motor remaining substantially stationary. The reverse pulse sequence then retracts the print head from the print medium by a fixed distance, adjusting the head gap to the proper size.
A problem with this device is that the continued sending of drive pulses to the stepping motor after the print head is stopped by contact with the print medium tends to make the stepping motor oscillate, creating a strong, unstable force that pushes the print head further forward, deforming the print medium and/or platen. The amount of deformation is variable because the oscillation is unstable, and because the number of pulses sent after contact differs depending on the thickness of the print medium. At the end of the forward pulse sequence, the print head is thus disposed an unpredictable distance forward of the original point of contact. The position to which the reverse pulse sequence moves the print head is unpredictable by the same amount. Applying fixed sequences of forward and reverse drive pulses to the stepping motor therefore fails to adjust the head gap with satisfactory accuracy.
Another problem is that driving the print head against the print medium may smudge the print medium.